A synchronous payment is a payment for which the final answer, approved or declined, comes back in the direct reply to the request that started it. The calling system sends the instruction, keeps the connection open for a second or two, and receives the result. A card authorization is the standard example: the merchant's request passes through the acquirer and network to the issuer and returns with a decision while the customer waits.
The opposite is an asynchronous payment, where the first reply only confirms that the instruction was received and the outcome arrives later by webhook notification or by polling. Bank debits, vouchers and many payouts work that way. Synchronous flows are simpler to build around, since an order can be confirmed immediately, but they depend on every party in the chain responding within the time limit. High latency or a timeout leaves the result unknown, which is why payment requests carry an idempotency key for safe retries.